1. Field of the Invention
This invention relates to a collision-type gas current pulverizer and a method for pulverizing powders, using a jet gas current (pressurized gas), and more particularly to a collision-type gas current pulverizer and a method for pulverizing powders for efficiently forming toners or color resin powders for the toners for use in the image formation by electrophotography.
2Related Background Art
A collision-type gas current pulverizer using a jet gas current is used to transport a powdery raw material with a jet gas current and allow the powdery raw material to collide with a colliding member, thereby pulverizing the powdery raw material by the force of collision.
A conventional collision-type, gas current pulverizer will be explained below, referring to FIG. 4.
A collision member 4 is provided against the outlet 13 of an acceleration pipe 43 connected to a compressed gas supply nozzle 2, and a powdery raw material is introduced into the acceleration pipe 43 from a powdery raw material hopper 1 in communication with the middle of the acceleration pipe 43 by suction generated by high speed flow of a high pressure gas supplied to the acceleration pipe 43 and ejected at the outlet 13 together with the high pressure gas, thereby subjecting the powdery raw material to collision with the collision member 4 and pulverizing the powdery raw material into finer powders through the collision. In order to pulverize the powdery raw material to a desired particle size, a classifier is provided between the powdery raw material hopper 1 and a discharge outlet 5 and is supplied with the powder from the pulverizer, and the classified coarse powders is supplied to the pulverizer through the powdery raw material hopper 1 and pulverized. The resulting pulverization product is returned to the classifier from the discharge outlet 5 to repeat the classification. Finer powders classified by the classifier are a finely pulverized product with the desired particle size.
However, in the foregoing prior art example, it is difficult to thoroughly disperse the powdery raw material introduced into the acceleration pipe by suction in the high pressure gas current, and thus the powder stream ejected at the outlet of the acceleration pipe contains a thick stream portion with a high powder concentration and a thin stream portion with a low powder concentration. Thus, the powder stream unevenly collides with the collision member counterposed to the outlet of the acceleration pipe, resulting in a decrease in the pulverization efficiency, which leads to a decrease in the powder-treating capacity. When the powder-treating capacity is to be increased in that state, the powder concentration in a pulverizing chamber 8 is partially increased, thereby making the powder stream uneven.
That is, the pulverization efficiency is lowered thereby.
Particularly in the case of resin-containing powders, a fusion product is unpreferably formed on the surface of the collision member.
In order to increase the pulverization efficiency of powder particles in the acceleration pipe 43, a pulverization pipe is proposed in Japanese Patent Publication No. 46-22778, which is provided with a high pressure gas feed pipe for ejecting a secondary high pressure gas at the position just before the outlet of acceleration pipe 43. The proposed pulverization pipe is directed to promotion of collision in the acceleration pipe and is a useful means for a pulverizer that conducts pulverization only in the acceleration pipe, but not a useful means for a collision-type, gas current pulverizer that conducts pulverization through collision with the colliding member, because the introduction of a secondary high pressure gas for promotion of collision in the acceleration pipe 43 impairs a transporting stream of the high pressure gas introduced from the compressed gas supply nozzle, thereby lowering the speed of the powder stream ejected at the outlet 13 of the acceleration pipe 43. Thus, the force of collision on the colliding member 4 is lowered and, also the pulverization efficiency is unpreferably lowered. In other words, a pulverizer with a good pulverization efficiency and a method for pulverization has been keenly desired.
On the other hand, toners and color resin powders for the toners for use in a process for forming an image by electrophotography usually contain at least a binder resin and a coloring agent or magnetic powders. The toners develop an electrostatically charged image formed on a latent image carrier, and the thus formed toner image is transferred onto a transfer material such as plain paper or a plastic film. The toner image on the transfer material is fixed to the transfer material by a fixing apparatus such as a heat fixing means, a pressure roller fixing means or a heat-pressure roller fixing means. Thus, the binder resin for use in the toners has such a characteristic as to undergo a plastic deformation when heat and/or a pressure is applied thereto.
Now, toners or color resin powders for the toners are prepared by fusion-kneading a mixture comprising at least a binder resin and a coloring agent or magnetic powders (and, if necessary, a third component) and cooling the fusion-kneaded product, followed by pulverization and classification. That is, the cooled product is usually subjected to coarse pulverization (or intermediate pulverization) by a mechanical, impact-type pulverizer (crusher) and the coarse pulverized powders are then subjected to fine pulverization by a collision-type, gas current pulverizer using a jet gas current.
When the pulverization capacity is to be increased in the conventional collision-type, gas current pulverized and the method for pulverization, as shown in FIG. 4, a fusion product is formed on the surface of colliding member 14, resulting in failure to stably produce the toners. Thus, an efficient collision-type, gas current pulverizer and a pulverization method for efficiently producing toners or color resin powders for the toners for use in the image formation by electrophotography, free from the foregoing problems, have been keenly desired.